Secondary temperature control for electric blankets



A. J. HUCK April 15, 1952 SECONDARY TEMPERATURE CONTROL FOR ELECTRIC BLANKETS Filed Aug. 17, 1948 r i l I I x I L r i INVENTOR. flZ/ifd cf. 15 amt/3i Patented Apr. 15, 1952 SECONDARY TEMPERATURE CONTROL FOR ELECTRIC BLANKETS Alfred J. Huck, St. Louis, Mo., assignor to Knapp- Monarch Company, St. Louis, Mo., a corporation of Delaware Application August 17, 1948, Serial No. 44,607

1 Claim.

This invention relates to a Warming blanket temperature control device which is of secondary nature as distinguished from a primary temperature control such as a cycling switch as shown in my copending applications, Serial No. 36,959 filed July 3, 1948, now abandoned, and Serial No. 155,073, filed April 10, 1950, or one of the type which serves as the manually adjustable or primary means of controlling the temperature of the blanket itself. My copending application Serial No. 41,796, filed July 31, 1948, now Patent No. 2,549,095, issued April 17, 1951, also shows a secondary control.

One object of the present invention is to provide a secondary temperature control which in the event of damage or other malfunctioning of the primary temperature control would operate to deenergize the warming blanket or the like and maintain it in the deenergized condition. The conventional method of obtaining such secondary and safety control is to provide a number of small thermostats and/ or fuses in the blanket proper which serve to limit the temperature of the blanket if the primary control fails to operate. However such arrangements have several obvious disadvantages. They require spliced connections to the blanket heating cable and such connections are vulnerable spots in the assembly. Thermostats and fuses are somewhat bulky and may be damaged when the blanket is flexed or otherwise handled in bed, and they detract from the ease of washing the blanket. In the case of temperature limiting thermostats there is no permanent deenergization of the blanket in the event of trouble in the primary control.

It is therefore another object of my invention to provide a secondary control which is not mounted in the blanket itself but may be mounted in the housing for the primary or main control as it is not appreciably sensitive in itself to ambient temperature.

A further object is to provide a secondary control which though not mounted in the blanket responds to the temperature of the blanket because of the heating cable in the blanket changing resistance as a result of increased temperature of the blanket itself.

It is therefore a further object to provide a secondary control which has either a differential relay or a relay operated from a transformer that has a differential primary winding in which one coil of the differential winding in either case opposes the magnetic efiect of the other coil substantially equally until such time as one of the coils suificiently overbalances the other as the blanket warms up excessively and thereby causes less current to flow through one of the coils which is in series with the heating element of the blanket.

With these and other objects in view, my invention consists in the construction, arrangement and combination of the various parts of my device whereby the objects contemplated are attained, as hereinafter more fully set forth, pointed out in my claim and illustrated in the accompanying drawings, wherein:

Figure 1 is an electro-diagrammatic view of a secondary control embodying my present invention and showing the use of an opposing coil differential relay.

Figure 2 is a cross sectional view through the relay itself, and

Figure 3 is a modification wherein a transformer having an opposing coil differential primary operates an ordinary relay from its secondary, the combination of transformer and relay being used in place of the differential relay of Figure 1.

On the accompanying drawing I have used the reference numeral 10 to indicate a service cord plug, the reference character PC to indicate a primary control, the reference character SC to indicate a secondary control and B a warming blanket or similar electrical appliance. Current supply wires 12 and 14 lead from the plug l0 to the primary control PC, the wire I4 having a control or off and on switch It therein if desired.

The primary control PC may be one of the type having a cycling switch and a power switch as shown in my copending application above referred to or may be any suitable type of control responding to temperature such as the temperature of the blanket B and/or temperature ambient to the primary control itself. Outlet wires I8 and 20 extend from the primary control to the secondary control and outlet wires 22 and 24 extend from the secondary control to the heatin element HE in the blanket B.

My secondary control SC as disclosed in Figures 1 and 2 consists of an opposing coil differential relay having a core 25 with a series or current coil S and a voltage or parallel coil V thereon. The series coil S is connected to the wires l3 and 22. The wire 24 is connected to a contact spring 30 of the relay SC which Spring is normally closed against a contact 32 connected with the wire 20. An armature 34 is provided for the relay and supports the contact sprin 30. This armature is adapted to be attracted by directions.

the core 26, the armature being pivoted by a leaf spring 35 to a relay frame 38 as shown in Figure 2 and the contact 32 being supported on and insulated from the frame. The armature 34 is preferably of the balanced type having a counterweight 38 or mounted with the armature down to prevent the possibility of the contacts 39--32 being separated by sudden movements or jarring of the secondary control SC.

The coils S and V are wound with the proper amount and size of wire, taking the resistance of the heating element HE into consideration so that each develops a magnetomotive force in an opposite direction and substantially equal to the other coil when the blanket B is at any normal operating temperature within the range of a normally functioning primary control PC. For instance the maximum temperature of the blanket under the action of the primary control may be 85 F.

The coil S is connected in series with the heating element HE and the coil V is connected across the line or in parallel with the heating element. The contacts 3fl32 are normally closed. and the opposing magnetomotive forces are balanced at average normal temperature such as 85 F. The coils of course are wound so that the current flows through them in opposite When the circuit is energized with the primary control functioning normally the relay armature 34 will not be attracted by the core 26 and in that case the secondary control is not yet functioning. The core 26 may be provided with a shading coil Ml to increase its efficiency and prevent hum.

The heating element HE may have a resistance value of about 60.2 ohms at 77 F. This ever and the ampere-turns therefore represent a decrease in magnetomotive force of the series coil S below that of the coil V suihcient to cause a medium-sensitive difierential-coil relay to operate. The relay can be designed and adjusted to provide relay contacts which open under such abnormal conditions to prevent damage by overheating of the blanket when the primary control malfunctions. Preferably the relay should become operative at a blanket temperature of,about 130 F. which is about 150 F. in the heating element HE and the range of operation may be adjusted or calibrated at the factory by means of a rheostat t24 l, the coil of which is indicated 42 and the movable arm is indicated 44. This rheostat and a resistor 28 are in series with the voltage coil V to produce the effect of changing the operating characteristics of the relay.

Once the coil V has overpowered the coil S in the manner described to cause the relay con-' tacts to open, the coil V will completely overpower the coil S since no current then flows through the coil S because the heating element HE is no longer in circuit withit. The armaturel34- will therefore remain attracted by the ,core 26 to prevent further energization of the heating element HE until such time as the trouble-with the blanket or in the primary control PC is remedied and the currenttothe wires 4 i8 and 2H shut oif at the switch It to permit the relay contacts to reclose. Thereafter the secondary control will function again in the nor mal manner.

Line voltage variation does not seriously affect the accuracy of the opposing coil differential relay as the magnetomotive force values of both the coils S and V are affected-in the same proportion. The relay coils, especially the coil V, should be wound with an alloy havin an extremely low temperature coefficient of resistance so that the coil temperature does not influence the relay pull-in value.

, To provide the 10 ampere-turn differential at 130 F. blanket temperature each coil should be designed to carry approximately ampereturns in the illustration given in the foregoing specification. These values however can be changed provided the electrical proportions remain approximately as outlined.

, .In Figure 3 I show a modified construction wherein the differential coils S and V form the primary coil of a transformer T, and a secondary coil 46 is provided to energize a relay coil it. At high blanket temperatures the difference in magnetomotive force causes an electromotive force in the secondary winding which is sufficient to actuate a small sensitive relay, the winding of which is the coil 48. Under normal blanket temperatures the coils S and V being wound in opposite directions on the transformer core substantially cancel each other as far as producing a flux in the core of the transformer is concerned but when the differential increases as much as might be represented by a blanket temperature up to F. then there is sufficient flux flow in the transformer core to produce by induction the necessary current in the coil 46 to operate the relay. When the relay opens then the heating element is out of circuit with the coil S and the current in the coil V predominates still more to keep the relay contacts open as in the case of Figure 1.

From the foregoing specification, it will be obvious that I have provided a secondary control which cuts ofi blanket energy completely as soon as an abnormal temperature condition is reached. Instead of permitting a continuous abnormal operation (assuming the primary control is faulty or the blanket is applied in an abnormal manner such as excessive folding of sections thereof) the protective relay permits the abnormal condition to exist only until the blanket attains a temperature of about 130 F. at which point the relay completely deenergizes the heating element and maintains the deenergized condition even though the blanket cools completely. After the trouble is remedied the relay can be reset by opening the supply circuit and then again closing the circuit.

The rheostat E2 i i serves as a calibrating adjustment for balancing all elements of the primary controlblanket-secondary control combination for proper operation of the secondary control at the desired limiting temperature. 'l?he arrangement is therefore simple and praction is always proper regardless of variation in supply line voltage. H In cases where two heating elements are in parallel in the blanket the secondary control automatically protects against operation if one of the two sections is open-circuited or partially shorted to the second section, which results in the full line current that normally passes through the coil S being increased or reduced and therefore overpowering the coil V or it overpowering the coil S and causing the armature 24 to pull in with positive snap action and the heating elements to be open circuited. Then of course the voltage coil V maintains the core 26 magnetized for maintaining the relay in the open-circuit position until it is reset. Thus magnetic difierential is taken advantage of for operating the secondary control with the positive snap action referred to being secured because of the series coil S being out of the circuit the instant the contact 30 is separated from the contact 32 and the coil V only being across the line (together with 28 and 42 of course).

A control of the character disclosed is relatively easy to calibrate and reliable in action.

Some changes may be made in the construction and arrangement of the parts of my device without departing from the real spirit and purpose of my invention, and it is my intention to cover by my claim any modified forms of structure or use of mechanical equivalents which may be reasonably included within its scope.

I claim as my invention:

In a secondary temperature control for a warning blanket or the like having a single heating element, said secondary temperature control comprising a pair of relay coils connected respectively in series and parallel with said single heating element so as to produce opposed magnetomotive forces in conjunction with said heating element for the series coil and independent of said heating element for the parallel coil, relay contacts in the heating element circuit, and an armature for said contacts, said contacts being normally closed and being opened by said relay coils upon the current in the series coil being either reduced in value because of increased resistance in said heating element resulting from excessive temperature of the blanket or increased in value because of decreased resistance in said heating element resulting from a short circuit of part or all of said heating element.

ALFRED J. HUCK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 919,402 Trumpler Apr. 27, 1909 1,183,814 Haagn May 16, 1916 1,569,928 Hartwig Jan. 19, 1926 1,687,652 Austin Oct. 16, 1928 1,959,690 Roth May 22, 1934 2,070,491 Park Feb. 9, 1937 2,158,133 Lemp May 16, 1939 2,184,741 Hartman Dec. 26, 1939 2,354,918 Kearsley Aug. 1, 1944 2,479,319 Crowley et a1 Aug. 16, 1949 2,549,095 Huck Apr. 17, 1951 OTHER REFERENCES McNairy: Abstract of application No. 725,852, published April 11, 1950. 

